Astronomy Astrophysics

A&A 398, 181–194 (2003) Astronomy DOI: 10.1051/0004-6361:20021634 & c ESO 2003 Astrophysics The dynamics of the nebula M1-67 around the run-away Wolf-Rayet star WR 124 M. V. van der Sluys1 andH.J.G.L.M.Lamers1,2 1 Astronomical Institute, Princetonplein 5, 3584 CC Utrecht, The Netherlands 2 SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands Received 18 July 2002 / Accepted 7 November 2002 Abstract. A new point of view on the dynamics of the circumstellar nebula M1-67 around the run-away Wolf-Rayet (WR) star WR 124 is presented. We simulated the outbursts of nebulae with different morphologies, to compare the results to the observed dynamical spectra of M1-67. We found that it has been interacting with the surrounding ISM and has formed a bow shock due to its high velocity of about 180 km s−1 relative to the local ISM. The star is about 1.3 parsec away from the front of this bow shock. The outbursts that are responsible for the nebula are assumed to be discrete outbursts that occurred inside this bow shock. The ejecta collide with this bow shock shortly after the outburst. After the collision, they are dragged away by the pressure of the ISM, along the surface of the bow shock. The bow shock is oriented in such way that we are looking from the rear into this paraboloid, almost along the main axis. Evidence for this is given firstly by the fact that the far hemisphere is much brighter than the near hemisphere, secondly by the fact that there is hardly any emission found with radial velocities higher than the star’s radial velocity, thirdly by the fact that the star looks to be in the centre of the nebula, as seen from Earth, and finally by the asymmetric overall velocity distribution of the nebula, which indicates higher radial velocities in the centre of the nebula, and lower velocities near the edges. We find evidence for at least two discrete outbursts that occurred inside this −1 bow shock. For these outbursts, we find expansion velocities of vexp ≈ 150 km s and dynamical timescales of about 0.8 and 2 × 104 yr, which are typical values for LBV outbursts. We therefore conclude that M1-67 originates from several outbursts that occurred inside the bow shock around WR 124, during an LBV phase that preceded the current WR phase of the star. Key words. stars: circumstellar matter – stars: individual: WR 124 – stars: mass-loss – stars: Wolf-Rayet – ISM: individual objects: M1-67 – ISM: jets and outflows 1. Introduction in Fig. 3. The nebula was first classified as an H region. After the discovery that the nebula has about the same radial velocity In this article, we describe our research on the dynamics of as WR 124, it was suggested that the nebula might be a plan- the Wolf-Rayet ring nebula M1-67. M1-67 is a bright nebula etary nebula (Minkowski 1946). However, the presence of a around the Wolf-Rayet (WR) star WR 124. The star has a high WR star and the N-enhancement and O-deficiency of the neb- −1 heliocentric velocity of almost 200 km s andisalsoknown ula suggest a WR ring nebula (Sirianni et al. 1998). The dis- as Merrill’s star (Merrill 1938) and 209 BAC. The star is clas- tance estimates also point in the direction of an ejected nebula, sified as a population I WN8 star (Bertola 1964) and is lo- so that M1-67 is now generally accepted as a Wolf-Rayet ring cated in the constellation Sagittarius. Distance estimates vary nebula. from about 4.5 kpc (Pismis & Recillas-Cruz 1979) to 6.5 kpc (Nugis & Lamers 2000). The star has a terminal wind velocity Though WR ring nebulae are not necessarily ring-shaped, of 710 km s−1 and a mass loss of 2.45 × 10−5 M yr−1.Itsmass they often exhibit a structure of arcs or rings. This suggests is estimated to be about 20 M and its luminosity 6 × 105 L that the nebulae may be created by discrete outburst events. (Nugis & Lamers 2000). It is generally thought that WR ring nebulae originate from a Luminous Blue Variable (LBV) stage of the central star, which The nebula M1-67 around WR 124 shows a clumpy struc- is supposed to precede the WR phase. ture, and most of the gas is concentrated in knots and filaments (Sirianni et al. 1998). An HST image of the nebula is displayed More than half of the LBVs have circumstellar nebulae (Nota & Clampin 1997). The different nebulae are very similar Send offprint requests to:H.J.G.L.M.Lamers, in terms of physical properties. The expansion velocities are in − e-mail: [email protected] the order of 50 to 100 km s 1, their sizes about 1 parsec, and the 182 M. V. van der Sluys and H. J. G. L. M. Lamers: The dynamics of the nebula M1-67 around the star WR 124 dynamical ages are in the order of 104 yr. The densities of the discusses the results of impacts of outbursts on the bow shock nebulae are generally found to be low (500 to 1000 cm−3)and surface. In Sect. 7 we summarise the results and present the the temperatures are in the range of 5000 to 10 000 K (Lamers conclusions of this study. et al. 2001). It is still a point of debate whether the LBV outbursts occur 2. Observations during a Red Supergiant (RSG) or a Blue Supergiant (BSG) phase. The RSG scenario is proposed by Stothers & Chin For this research, we used the following three sets of observa- (1993, 1996), who suggest that the ejection of mass occurs only tional data. as a single event during a brief RSG phase. They explain the enhanced abundances of heavy elements of the ejecta by con- 2.1. Long-slit spectra vective mixing in the RSG envelope (Stothers & Chin 1993, 1996). The first dataset we used, is velocity information from long slit On the other hand, Langer et al. (1994) suggest that after spectra we obtained from A. Nota, published in Sirianni et al. the star has left the main sequence, it moves red-ward in the (1998). These data consist of 13 long slit spectra, taken with the Hertzsprung-Russel diagram (HR diagram) and the expand- ESO Multi Mode Instrument (EMMI) at the 3.5 m NTT in La ing envelope becomes unstable, so that the star starts to de- Silla. Each slit is positioned over the nebula in the east-west di- velop extreme mass loss. This mass loss may be as high as rection, at constant declination. The declinations of the slits lie 5 × 10−3 M yr−1 and is observed as the LBV ejecta. They ex- between –30.82 and +24.69 relative to the star’s declination. plain the chemically enriched ejecta by rotation induced mix- In total, 413 good data points (right ascension, declination and ing. These eruptions therefore take place when the star is a radial velocity) were derived from the spectra, which formed BSG and prevent it from becoming a RSG (Langer et al. 1994; the input for our study. These data points have been plotted in Lamers et al. 2001). Fig. 1. The research of the chemical composition of LBV-ejecta by Lamers et al. (2001) also indicates that the LBV eruptions oc- 2.2. Fabry-Perot´ images cur during a BSG phase. They suggest that the LBV outbursts are induced by the rapid, near-critical rotation of the star. In The second dataset are Fabry-Pérot (FP) images obtained from their scenario, the star is also being prevented from becoming Grosdidier et al. (1999). The FP images are shown in Fig. 2. a RSG by the mass loss. However, they point out that if a mas- These 30 images were made in August 1996, using CFHT-SIS, sive, optically thick shell is being expelled from the star, it will with theetalon ´ of the UniversitéLaval,Québec, Canada. Each cool as it expands and the physical conditions will temporarily image was taken in Hα, at a slightly different wavelength, so be similar to that in the outer layers of a RSG, so that the for- that it displays the emission at a certain radial velocity. These mation of dust can also happen in this case. This mechanism observations give a much more detailed view of the velocity explains the observed Humphreys-Davidson limit, that depicts distribution than the long slit spectra. Each image consists of the lack of RSGs with luminosities higher than 6 × 105 L. 100 × 100 pixels. From the darkest points, we used the coor- The goal of our research is to disentangle the geometry and dinates (x, y, v) in the same way as the data points from the dynamics of the nebula M1-67. In order to do so, we create long slit spectra. A few thousand points were used. Note that different numerical models, of which the output is compared the southern part of the nebula, more than approximately 20 to available observations. Firstly, we present the observational south of the star, is missing (compare to Fig. 3 and see Fig. 8b) data we use for this study in Sect. 2. We will then discuss our due to deteriorating seeing during the observations (Grosdidier models for freely expanding outbursts in Sect. 3. The reason et al. 1999). why we let these models expand freely is that the O-star that precedes a WR star blows a bubble of typically 30 pc in the 2.3.

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